1. Field of the Disclosure
The present disclosure relates to a chemically prepared toner formulation for use in electrophotography, and more specifically, to a polyester-based emulsion aggregation toner formulation.
2. Description of the Related Art
Electrophotography is a widely used printing technique that includes generation of an image on an image-receiving medium using a toner. More specifically, the technique includes a transfer of a specific toner to the image-receiving medium with the help of electrostatic charges. Suitable examples of the image-receiving medium include, but are not limited to, paper, plastic, and textile. The technique of electrophotography is broadly used in photocopying machines, laser printers, Light-Emitting Diode (LED) printers, and the like.
In general, toners used in electrophotographic printers are of two types, namely, milled toners and chemically prepared toners or chemically processed toners (CPTs). The milled toners may be produced either by a mechanical milling/grinding process or by a jet milling process. Suitable examples of the milled toners include, but are not limited to, mechanically milled toners and jet-milled toners.
Several types of CPTs include, but are not limited to, suspension polymerization toners (SPTs), emulsion aggregation toners (EATs)/latex aggregation toners (LATs), toners made from a dispersion of pre-formed polymer in solvent (DPPTs) and CPTs made from a “chemical milling” method.
Typically, use of CPTs is preferred over use of the milled toners as the CPTs provide better print quality, better toner transfer efficiency and lower torque properties for various components of the electrophotographic printers, such as a developer roll, a fusing belt and a charge roll. Further, chemical techniques employed for preparing the CPTs allow for manufacture of toner particles with small sizes for better fusing and printing properties as opposed to the milled toners. Furthermore, size distribution and shape of the toner particles may be well controlled while preparing the CPTs for improved toner properties as opposed to the milled toners.
More specifically, the EATs have shown to have several advantages over the milled toners and other types of the CPTs. Such advantages include, but are not limited to, manufacturing of toner particles with a small particle size and narrow particle size distribution, such as from about 3 to about 10 micrometers (μm), and achieving an optimized shape of the toner particles, such as a potato-like shape. Such optimized shape of the toner particles is required for proper and efficient cleaning of the CPTs from various components of the electrophotographic printers, such as a developer roll, a charge roll and doctoring blades, in order to prevent filming/unwanted deposition of the CPTs over the components. Further, the optimized shape of the toner particles enables for proper toner transfer properties during an electrophotographic printing operation.
In a typical process used for preparing an EAT, emulsion aggregation is carried out in an aqueous system resulting in good control of both size and shape of toner particles. Further, preparation of the EAT usually involves components, such as latex binder, one or more colorants and wax. More often than not, a styrene-acrylic copolymer latex binder is used as the latex binder in emulsion aggregation process. However, use of the styrene-acrylic copolymer latex binder allows the EAT to either have a good toner fusing property with poor shipping/storage properties or have a poor toner fusing property with good shipping/storage properties. Further, such an EAT is known to exhibit poor mechanical properties in terms of durability and resistance to filming of the various components of the electrophotographic printers.
Alternatively, use of polyester binder resins has proven to be advantageous as opposed to the styrene-acrylic copolymer latex binder and other latex binders for preparing toners for electrophotography. However, the polyester binder resins are usually employed for preparing milled toners, and have rarely been employed to prepare CPTs. The polyester binder resins are manufactured using condensation polymerization technique, which is a time-consuming technique due to involvement of long polymerization cycles. Accordingly, the polyester binder resins are not preferably adapted for the emulsion aggregation process, as the process is then confined to the use of polyester binder resins having polyester polymers with low-to-moderate molecular weights. This results in preparation of polyester-based toner formulations with limited toner fusing and printing properties.
Further, the polyester binder resins are not capable of properly dispersing in the aqueous system, i.e. water, during an emulsion aggregation process due to their polar nature, pH sensitivity and gel content. More specifically, some polyester binder resins form unstable emulsions when used in the emulsion aggregation process, and thereby yield polyester-based toner formulations with poor toner properties. In addition, due to formation of unstable emulsions, it is generally not possible to use polyester binder resins with a low acid value, for example, an acid value less than about 10, and/or gel content more than about 5 percent.
However, with advancement in toner manufacturing technology, it has become possible to obtain stability in emulsions formed using the polyester binder resins. This has been achieved by dissolving the polyester binder resins in an organic solvent and then performing a phase-inversion process where water is added slowly in a drop-wise manner. Subsequently, the organic solvent is evaporated for allowing the polyester binder resins to form stable emulsions (hereinafter referred to as “polyester resin emulsions”). Suitable examples of the organic solvents include, but are not limited to, ethyl acetate, methyl ethyl ketone (MEK), methylene chloride, chloroform and tetrahydrofuran (THF). Although, polyester resin emulsions employed in conventional polyester-based toner formulations, as obtained using the emulsion aggregation process, have shown good compatibility with colorants and wax, the conventional polyester-based toner formulations so obtained have shown limited toner fusing and printing properties. Such limited toner fusing and printing properties are associated with the use of a narrow range of the polyester polymers in the polyester binder resins. More specifically and as mentioned above, polyester polymers with low-to-moderate molecular weights and low-to-moderate molecular weight distributions have usually been employed in the polyester binder resins due to time limitations associated with complex condensation polymerization techniques.
Further, a polyester binder resin formed from a polyester polymer, which is free of cross-linking, using condensation polymerization has a low molecular weight distribution with a theoretical value of about 2. Accordingly, a polyester-based toner formulation prepared using such polyester polymer in a polyester binder resin typically has a poor fusing performance and durability unless the molecular weight distribution of the polyester binder resin is broadened using a cross-linking agent. In general, cross-linked polyester binder resins are required to obtain adequate durability and shipping/storage performance. Further, the cross-linked polyester binder resins are required to obtain good fuse grade performance for a contact development process where a developer roll is rotatably disposed in contact with a photosensitive member (such as a photoconductive drum) and the developer roll applies a layer of a polyester-based toner formulation directly to a surface of the photosensitive member. However, when using a cross-linked polyester binder resin in an emulsion aggregation process, it becomes extremely difficult to break the cross-links for proper dissolution of the polyester binder resin in an organic solvent in order to form a stable emulsion for preparing a polyester-based toner formulation with good toner properties.
Additionally, it is quite difficult to prepare a polyester-based toner formulation using a combination of two or more different polyester binder resins and/or different polyester resin emulsions by the conventional emulsion aggregation process. More specifically, polyester binder resins that include different monomers, different acid values, different softening temperatures and/or different melt viscosities, tend to agglomerate differently, thereby preventing the emulsion aggregation process from achieving polyester-based toner formulations with narrow particle size distributions for better fusing and printing properties.
Accordingly, there is a need for preparing an emulsion aggregation toner formulation with a broad molecular weight distribution. Further, the emulsion aggregation toner formulation should be capable of exhibiting good fusing properties without compromising shipping/storage performance and durability thereof. Furthermore, the emulsion aggregation toner formulation should be capable of exhibiting good resistance to filming various components of electrophotographic printers and good gloss when used for printing on an image-receiving medium.